1. Field of the Invention
The present invention relates to a method of increasing a free carrier concentration in a semiconductor substrate. More particularly, the present invention relates to a method of heating a semiconductor substrate using a laser that is absorbed in the semiconductor substrate due to an increased free carrier concentration in the semiconductor substrate.
2. Description of the Related Art
As ULSI devices are scaled down below 100 nm, a highly-doped ultra shallow junction is required for high performance devices with a short channel. In order to achieve a shallow junction, rapid annealing (i.e., on the order of milliseconds) technology is required. A laser annealing method has been proposed as an ideal technique for this purpose, however, it is well known that there are detrimental effects associated with laser annealing, such as a pattern effect or a diffraction effect, that prevent it from being adopting in an ultra large scale integration (ULSI) process. A pattern effect, for example, is caused when a laser used in the annealing process has a wavelength that is similar to a device size. The pattern effect results in adjacent regions being heating differently.
One way to overcome the pattern effect is to use a laser having a wavelength that is significantly longer, i.e., larger, than a device size, for example, an infrared laser. An example of an infrared laser is a CO2 laser, which is relatively common, is relatively inexpensive, has a high power, and has a relatively long wavelength of 10 μm. More specifically, a CO2 laser has a maximum power of about 3000 W. By comparison, most lasers typically have a power of less than 10 W.
Conventionally, CO2 lasers have not been suitable for use in semiconductor processing because, for example, a CO2 laser having a wavelength of 10 μm is not absorbed by a semiconductor substrate. In fact, the CO2 laser simply passes through the substrate without being absorbed. More specifically, as may be seen in FIG. 1, an absorption coefficient of silicon, for example, is virtually zero (0) at wavelengths of 1.1 μm and above. FIG. 1 is a graph of absorption coefficient versus wavelength for an undoped silicon wafer.
FIG. 2 is a graph of absorption coefficient (α) versus wavelength (λ) at varying doping levels of a p-type silicon (Si) wafer at a temperature of 300 K. FIG. 2 additionally is a graph showing a relationship between free carrier concentration and absorption coefficient.
In FIG. 2, a first hole concentration (line 1) is 4.6×1017 cm−3, a second hole concentration (line 2) is 1.4×1018 cm−3, a third hole concentration (line 3) is 2.5×1018 cm−3, and a fourth hole concentration (line 4) is 1.68×1019 cm−3. As may be seen in FIG. 2, with a free carrier concentration of approximately 1018 cm−3, an absorption coefficient at a wavelength of 10 μm is about 300 cm−1. At an absorption coefficient of about 300 cm−1, a silicon substrate can be sufficiently heated by a laser having a wavelength of 10 μm. Thus, a silicon substrate can be effectively heated by a laser having a relatively long wavelength if the concentration of the free carrier is sufficiently increased in the silicon substrate.
Referring back to FIG. 1, it may also be noted that a thickness of a light absorbing layer xj decreases as an absorption coefficient increases. Accordingly, an annealing depth may be controlled by adjusting a wavelength of the incident light.
The present invention relates to a laser annealing method using an infrared laser, which causes only negligible defects, such as a pattern effect and a diffraction effect. In general, infrared laser beams are not absorbed by undoped Si wafers. The present invention increases an absorption coefficient of a semiconductor substrate by increasing a free carrier concentration in the semiconductor substrate, thereby making heating with an infrared laser possible. Moreover, the present invention provides a localized heating method to heat a selected region of a semiconductor wafer in the micro scale, which has been impossible with conventional heating technology.
The embodiments of the present invention are able to increase free carrier concentration to a level of 1018 cm−3 in order to increase an absorption coefficient to a sufficient range of approximately 103 cm−1, thereby allowing use of a 10 μm wavelength CO2 laser to heat the semiconductor substrate.